Designed for sealed or ported...explained! - Car Audio | DiyMobileAudio.com | Car Stereo Forum

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Old 09-16-2011   #1
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Default Re: Designed for sealed or ported

Quote:
Originally Posted by captainscarlett View Post
I want to get into a bit of box building for the sake of a regaining some boot space. And as people seem to have a distinct disdain for prefab enclosures, i thought i'd start to do a bit of reading up to make sure I have at least a basic idea of what's going on.



Yet on another forum you get this advice, which seems contradict some of the suggestions for sealed or ported enclosures:




So who or which advice do i believe? or am i looking at this wrong?


So if I'm to take the above advice, this would be in a ported box ...




The DD, of which I have all three enclosed subs, would be the biggest contender for a sealed box .. (but having a little experience of DD I wouldn't even try it)




.... and this would have to be the biggest contender for a ported box and a sealed box wouldn't even get a look in!
they're both right and they're both wrong.

Here's the deal...go get a cup of coffee...

The Qts of the speaker indicates the shape of the roll-off. A Qts of .707 is considered ideal for infinite baffle because provides the best compromise between low frequency extension and transient response. So, let's say you have a woofer and you want to build a sealed box. Let's also say you have a TARGET Qtc (total Q of the box and woofer) of .707 and your woofer has a Qts of .5

The box volume will be calculated to provide an "aid" to the speaker's suspension (makes the suspension stiffer) so that the Qtc will be raised to the target of .707. If the woofer has a very compliant suspension (low Q) then the box will have to contribute more restoring force, so it will have to be small. If the woofer has a stiff suspension (higher Q) then the box will have to contribute less restoring force, so it will have to be bigger. If the Qts of your woofer is higher than the target Qtc, then the target isn't achievable.

So, the Q of the woofer determines the volume of the box for a given target Qtc. The low frequency extension is determined by the woofer's resonance frequency, Fs. The Fs is also raised in the sealed box iin the same proportion as the increase in Qts to Qtc.

So, a woofer with a really low Qts and a low Fs is well suited to a small sealed box SO LONG AS THE VOICE COIL IS LONG. Sealed boxes require much more excursion from the speaker than vented boxes do. A woofer with a short coil is not well suited to a sealed box unless it will be driven with very low power.

A vented box is more complicated. It's a combination of a sealed box and an additional resonance. A low Q woofer designed for a sealed box will work just fine in a vented box, but the additional coil necessary for the sealed box will be wasted in the vented box and its mass makes for a less efficient system than a woofer with the same Q and less mass.

Anyway, the box volume for a vented box is calculated to provide a target response similar to the sealed box. Usually the volume is a bit bigger because we want a lower Qtc (which doesn't really exist in the vented arrangement, but we'll use it as an example).

The port is basically a speaker that's designed to play loudly at one note. Its response is a peak. We design the port to have a response that compliments the response of the woofer. We hear the sum of the output of the woofer and the output of the port. If we increase the tuning frequency, we have an area where the port's response and the woofer's response have a small gap--that creates a peak in the response in those frequencies. If we move the port to a lower frequency, we have a dip in the frequencies in between. Usually, the resonance frequency is chosen to extend the low frequency response as low as possible while maintaining flat response. For small bookshelf-style home speakers, a little bump is often helpful in providing some additional bass.

At the frequency where the box is tuned, the port plays, but the woofer hardly moves. This is because the acoustic impedance (pressure) inside the box is much higher. Yes, the pressure inside a ported box is HIGHER (at the tuned frequency) than in a sealed box. Below the frequency where the box is tuned, there is much LESS pressure than in a sealed box--hence the need for a subsonic filter. So, at the low frequencies where the box is tuned, the woofer doesn't move much, so we don't need a heavy long coil. That makes it easier to make a more efficient woofer, since we don't need so much moving mass.

OK, so what does all of this mean?

1. If you're going to use lots of power and a sealed box, then you need a woofer with a long coil.

2. If you're going to use a vented box, a woofer with a shorter coil will be fine.

3. If you use a woofer designed for a small sealed box (low Q) in a vented box, the box volume requirement will be small and the port frequency requirement will be low. That means the port will be very long and the box will be difficult to build because the port will be difficult to fit in the box.

4. If you use a woofer with a short coil, designed for a vented box, in a sealed box with high power, the woofer will run out of coil and you'll hear distortion. Because the condition that produces the distortion is symmetrical (coil leaves the gap in both directions), the distortion will be mostly odd-order, which sounds nasty. "Brap Brap Brap".

So, what's the conclusion?
Your power requirement and choice of box type determines whether a long coil (woofer with high Xmax) is necessary. Almost all woofers will work in a sealed or vented box, so long as the woofer's Qts is lower than your target Qtc. A woofer with a Qts higher than .707 will have a peak in its response NO MATTER WHAT KIND OF BOX YOU USE. The rest is a matter of compromising between box size, power required to hit a target SPL and required low frequency extension.

When you put the box in the car, the transfer function (car's frequency response) will be added to the response of the woofer. this will produce roughly a 12dB per octave increase in level as frequency is reduced starting at about 50 or 60 Hz, depending on the size of the car. A vented box will give you a big peak and a sealed box will not. If you have an EQ, then you can reduce the peak by reducing the power the amp has to provide at those frequencies. I think a vented box and EQ is always the best way to go, so long as you can afford the space and the EQ.
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Old 10-05-2012   #2
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Default Re: Designed for sealed or ported...explained!

I would say very well put coming from a gentleman that works for a company, whose speakers can be found in most colesieums you would visit for a concert.(Simply look up).

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Old 10-14-2012   #3
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Default Re: Designed for sealed or ported...explained!

This can also help people understand how IB (infinite baffle) works, since it is generally a giant sealed. Though with IB you often have the option of going with more cone area to offset the use of lower xmax subs that can keep efficiency higher/distortions lower...since you only need mount them and not box them.

Pioneer 880PRS~boston comps/alpine coax on Kappa 4 Z~back to Alpine mrd-m500 v12 on pyle 15s IB. But now have a new car, this will take a while to figure out.
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Old 10-24-2012   #4
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Default Re: Designed for sealed or ported...explained!

Is there a link to visit to determine what is considered a long voice coil for a given speaker diameter? Same with determining what is low or high for Q & Fs numbers?

I ask because I built a small sealed trunk corner fiberglass enclosure (~.6 to .7 cf) for a Pioneer TS-W259D4 sub before realizing I needed to consider all of this LOL...

It performs well except for punch. It's very weak in the bass drum punch area and I'm trying to nail down why. I still plan to fill it with some fiberglass insulation to see if that helps with any standing wave problems, but I'm curious if a different sub may fit the space better, ex JL Audio 10W3v3.

TS-W259D4 Specs
Free Air Resonance (Fs) 48.0 Hz
Electrical “Q” (Qes) 1.09
Mechanical “Q” (Qms) 11.64
Total Speaker “Q” (Qts) 1.00
Equivalent Compliance (Vas) 0.52 cu ft / 14.63 L
One-Way Linear Excursion (Xmax)* 0.47 in / 12.0 mm
Efficiency (1 W)** 92 dB SPL

JL Audio 10W3v3 Specs
Free Air Resonance (Fs) 32.97 Hz
Electrical “Q” (Qes) 0.533
Mechanical “Q” (Qms) 7.027
Total Speaker “Q” (Qts) 0.495
Equivalent Compliance (Vas) 1.136 cu ft / 32.17 L
One-Way Linear Excursion (Xmax)* 0.550 in / 14.0 mm
Efficiency (1 W / 1 m)** 85.37 dB SPL

Last edited by Married_Man; 10-24-2012 at 11:46 AM..
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Old 12-06-2012   #5
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Default Re: Designed for sealed or ported...explained!

So for a small sealed box you want a driver with a low Qts, low fs, and high xmax. Why does Fi offer a high Qts option on their Q subwoofers for a small sealed application? I'm just trying to get a better understanding of all of this.

And what does Fi change to achieve this?

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Old 12-06-2012   #6
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Default

Quote:
Originally Posted by mitchjr View Post
So for a small sealed box you want a driver with a low Qts, low fs, and high xmax. Why does Fi offer a high Qts option on their Q subwoofers for a small sealed application? I'm just trying to get a better understanding of all of this.

And what does Fi change to achieve this?
The enclosure will raise Q, when we get beyond Qtc 0,8 there will be a response peak above -f3, the higher Q the higher the peak will be. Some might prefer the sound of such an enclosure but if you have a DSP you can EQ the peak out and get a more power efficient design.

A high Q woofer generally have flatter response in IB configurations.

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Old 12-06-2012   #7
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Default Re: Designed for sealed or ported...explained!

You want to get confused, take a look at the Pioneer shallow subs' T/S specs.

Q is just where the sub is 'happy' at. Starts with Fs is where its tuned but the qts tells you how close to Fs it is happy. High qts will be happy down by Fs, so it will play (IB they are tested at) well low and naturally have more output higher...so it will be flatter IB. Lower qts means it will not be as happy down there, which leaves better response up higher so its weak on bass. Until you put it in a box and push the Q up, then it gets happy a little higher frequency down low than the IB sub did. But if your Fs was in the 20s it will be happy in the 30s which is ok in cars. The IB sub will be happy about 25 and up if Fs is 20.

The easiest way to see what is going on is to take some different drivers and model them. Even if you take an average sub and make up a fake TS with higher qts and lower, change the Fs, etc., then compare them all.

Pioneer 880PRS~boston comps/alpine coax on Kappa 4 Z~back to Alpine mrd-m500 v12 on pyle 15s IB. But now have a new car, this will take a while to figure out.

Last edited by sqshoestring; 12-06-2012 at 08:16 PM..
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Old 05-27-2013   #8
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Default Re: Designed for sealed or ported

Quote:
Originally Posted by Andy Wehmeyer View Post
they're both right and they're both wrong.

Here's the deal...go get a cup of coffee...

The Qts of the speaker indicates the shape of the roll-off. A Qts of .707 is considered ideal for infinite baffle because provides the best compromise between low frequency extension and transient response. So, let's say you have a woofer and you want to build a sealed box. Let's also say you have a TARGET Qtc (total Q of the box and woofer) of .707 and your woofer has a Qts of .5

The box volume will be calculated to provide an "aid" to the speaker's suspension (makes the suspension stiffer) so that the Qtc will be raised to the target of .707. If the woofer has a very compliant suspension (low Q) then the box will have to contribute more restoring force, so it will have to be small. If the woofer has a stiff suspension (higher Q) then the box will have to contribute less restoring force, so it will have to be bigger. If the Qts of your woofer is higher than the target Qtc, then the target isn't achievable.

So, the Q of the woofer determines the volume of the box for a given target Qtc. The low frequency extension is determined by the woofer's resonance frequency, Fs. The Fs is also raised in the sealed box iin the same proportion as the increase in Qts to Qtc.

So, a woofer with a really low Qts and a low Fs is well suited to a small sealed box SO LONG AS THE VOICE COIL IS LONG. Sealed boxes require much more excursion from the speaker than vented boxes do. A woofer with a short coil is not well suited to a sealed box unless it will be driven with very low power.

A vented box is more complicated. It's a combination of a sealed box and an additional resonance. A low Q woofer designed for a sealed box will work just fine in a vented box, but the additional coil necessary for the sealed box will be wasted in the vented box and its mass makes for a less efficient system than a woofer with the same Q and less mass.

Anyway, the box volume for a vented box is calculated to provide a target response similar to the sealed box. Usually the volume is a bit bigger because we want a lower Qtc (which doesn't really exist in the vented arrangement, but we'll use it as an example).

The port is basically a speaker that's designed to play loudly at one note. Its response is a peak. We design the port to have a response that compliments the response of the woofer. We hear the sum of the output of the woofer and the output of the port. If we increase the tuning frequency, we have an area where the port's response and the woofer's response have a small gap--that creates a peak in the response in those frequencies. If we move the port to a lower frequency, we have a dip in the frequencies in between. Usually, the resonance frequency is chosen to extend the low frequency response as low as possible while maintaining flat response. For small bookshelf-style home speakers, a little bump is often helpful in providing some additional bass.

At the frequency where the box is tuned, the port plays, but the woofer hardly moves. This is because the acoustic impedance (pressure) inside the box is much higher. Yes, the pressure inside a ported box is HIGHER (at the tuned frequency) than in a sealed box. Below the frequency where the box is tuned, there is much LESS pressure than in a sealed box--hence the need for a subsonic filter. So, at the low frequencies where the box is tuned, the woofer doesn't move much, so we don't need a heavy long coil. That makes it easier to make a more efficient woofer, since we don't need so much moving mass.

OK, so what does all of this mean?

1. If you're going to use lots of power and a sealed box, then you need a woofer with a long coil.

2. If you're going to use a vented box, a woofer with a shorter coil will be fine.

3. If you use a woofer designed for a small sealed box (low Q) in a vented box, the box volume requirement will be small and the port frequency requirement will be low. That means the port will be very long and the box will be difficult to build because the port will be difficult to fit in the box.

4. If you use a woofer with a short coil, designed for a vented box, in a sealed box with high power, the woofer will run out of coil and you'll hear distortion. Because the condition that produces the distortion is symmetrical (coil leaves the gap in both directions), the distortion will be mostly odd-order, which sounds nasty. "Brap Brap Brap".

So, what's the conclusion?
Your power requirement and choice of box type determines whether a long coil (woofer with high Xmax) is necessary. Almost all woofers will work in a sealed or vented box, so long as the woofer's Qts is lower than your target Qtc. A woofer with a Qts higher than .707 will have a peak in its response NO MATTER WHAT KIND OF BOX YOU USE. The rest is a matter of compromising between box size, power required to hit a target SPL and required low frequency extension.

When you put the box in the car, the transfer function (car's frequency response) will be added to the response of the woofer. this will produce roughly a 12dB per octave increase in level as frequency is reduced starting at about 50 or 60 Hz, depending on the size of the car. A vented box will give you a big peak and a sealed box will not. If you have an EQ, then you can reduce the peak by reducing the power the amp has to provide at those frequencies. I think a vented box and EQ is always the best way to go, so long as you can afford the space and the EQ.
So to quantify what a long voice coil looks like for a sealed enclosure- are we talking about the need for one of those rocket launcher looking subwoofers? e.g. with nearly as much mounting depth as the nominal size of the driver and with a 1-way excursion capability of 29mm for a 12" driver?

Would it be safe to say a 12" driver with a 1-way xmax limit of 15mm would not qualify as "long"?

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Old 05-27-2013   #9
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Default Re: Designed for sealed or ported

Quote:
Originally Posted by avanti1960 View Post
So to quantify what a long voice coil looks like for a sealed enclosure- are we talking about the need for one of those rocket launcher looking subwoofers? e.g. with nearly as much mounting depth as the nominal size of the driver and with a 1-way excursion capability of 29mm for a 12" driver?

Would it be safe to say a 12" driver with a 1-way xmax limit of 15mm would not qualify as "long"?
If you're using a 500-1000 watt amp, it's probably fine. If you're using some huge amp, then you'll probably want one of those rocket launcher-looking subs. 15mm is pretty long. 7mm is not.
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Old 06-17-2014   #10
 
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Default Re: Designed for sealed or ported...explained!

A sealed cabinet is considered a punchier, more accurate sound.

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Last edited by ethelbritt; 06-18-2014 at 02:32 AM..
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Old 06-17-2014   #11
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Default Re: Designed for sealed or ported...explained!

Quote:
Originally Posted by ethelbritt View Post
A sealed cabinet is considered a punchier, more accurate sound.
no its not. not anymore than an 8" sub is more punchy than a 15". this is a myth that has been disproved so many times its not funny.

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Old 06-17-2014   #12
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Default Re: Designed for sealed or ported...explained!

A sealed box always sounds cleaner......always

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Old 06-17-2014   #13
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Default Re: Designed for sealed or ported...explained!

^^ No it doesn't. In fact, a vented box can be made to play with MUCH less distortion and MUCH more output.
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Old 06-17-2014   #14
 
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Default Re: Designed for sealed or ported...explained!

Quote:
Originally Posted by minbari View Post
no its not. not anymore than an 8" sub is more punchy than a 15". this is a myth that has been disproved so many times its not funny.
Ugg that misconception has been my biggest pet peeve in audio.....
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Old 06-18-2014   #15
 
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Default Re: Designed for sealed or ported...explained!

Quote:
Originally Posted by Andy Wehmeyer View Post
^^ No it doesn't. In fact, a vented box can be made to play with MUCH less distortion and MUCH more output.
Listen to The Man God damn it. He is right on money with this one, I stopped using sealed at the age 18. I alway being able bass reflex (ported) if you prefer, sounds better then sealed even with "designed for sealed" drivers. I still have Klipschorns (sealed) and that is as far as I stretch my sealed collection.
Sealed has one purpose - make the box smaller, everything else justified by that.
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Old 06-18-2014   #16
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Default Re: Designed for sealed or ported...explained!

Quote:
Originally Posted by Andy Wehmeyer View Post
^^ No it doesn't. In fact, a vented box can be made to play with MUCH less distortion and MUCH more output.
I'm not talking about how it measures, but how it sounds. I spent 6 months trying to get a ported box sound like my 1.25 cuft box. Tried different box/ports, eq, timing etc etc. No go. Yes a ported is louder and yes it has lower measured 2nd order. My take is that some second order at the lower end adds warmth to the sound.

Ported subs can also sound tight and punchy, that depends on the overall response. But the two do sound different. I'm just not qualified to say why.

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Default Re: Designed for sealed or ported

Quote:
Originally Posted by Andy Wehmeyer View Post
they're both right and they're both wrong.

Here's the deal...go get a cup of coffee...

The Qts of the speaker indicates the shape of the roll-off. A Qts of .707 is considered ideal for infinite baffle because provides the best compromise between low frequency extension and transient response. So, let's say you have a woofer and you want to build a sealed box. Let's also say you have a TARGET Qtc (total Q of the box and woofer) of .707 and your woofer has a Qts of .5

The box volume will be calculated to provide an "aid" to the speaker's suspension (makes the suspension stiffer) so that the Qtc will be raised to the target of .707. If the woofer has a very compliant suspension (low Q) then the box will have to contribute more restoring force, so it will have to be small. If the woofer has a stiff suspension (higher Q) then the box will have to contribute less restoring force, so it will have to be bigger. If the Qts of your woofer is higher than the target Qtc, then the target isn't achievable.

So, the Q of the woofer determines the volume of the box for a given target Qtc. The low frequency extension is determined by the woofer's resonance frequency, Fs. The Fs is also raised in the sealed box iin the same proportion as the increase in Qts to Qtc.

So, a woofer with a really low Qts and a low Fs is well suited to a small sealed box SO LONG AS THE VOICE COIL IS LONG. Sealed boxes require much more excursion from the speaker than vented boxes do. A woofer with a short coil is not well suited to a sealed box unless it will be driven with very low power.

A vented box is more complicated. It's a combination of a sealed box and an additional resonance. A low Q woofer designed for a sealed box will work just fine in a vented box, but the additional coil necessary for the sealed box will be wasted in the vented box and its mass makes for a less efficient system than a woofer with the same Q and less mass.

Anyway, the box volume for a vented box is calculated to provide a target response similar to the sealed box. Usually the volume is a bit bigger because we want a lower Qtc (which doesn't really exist in the vented arrangement, but we'll use it as an example).

The port is basically a speaker that's designed to play loudly at one note. Its response is a peak. We design the port to have a response that compliments the response of the woofer. We hear the sum of the output of the woofer and the output of the port. If we increase the tuning frequency, we have an area where the port's response and the woofer's response have a small gap--that creates a peak in the response in those frequencies. If we move the port to a lower frequency, we have a dip in the frequencies in between. Usually, the resonance frequency is chosen to extend the low frequency response as low as possible while maintaining flat response. For small bookshelf-style home speakers, a little bump is often helpful in providing some additional bass.

At the frequency where the box is tuned, the port plays, but the woofer hardly moves. This is because the acoustic impedance (pressure) inside the box is much higher. Yes, the pressure inside a ported box is HIGHER (at the tuned frequency) than in a sealed box. Below the frequency where the box is tuned, there is much LESS pressure than in a sealed box--hence the need for a subsonic filter. So, at the low frequencies where the box is tuned, the woofer doesn't move much, so we don't need a heavy long coil. That makes it easier to make a more efficient woofer, since we don't need so much moving mass.

OK, so what does all of this mean?

1. If you're going to use lots of power and a sealed box, then you need a woofer with a long coil.

2. If you're going to use a vented box, a woofer with a shorter coil will be fine.

3. If you use a woofer designed for a small sealed box (low Q) in a vented box, the box volume requirement will be small and the port frequency requirement will be low. That means the port will be very long and the box will be difficult to build because the port will be difficult to fit in the box.

4. If you use a woofer with a short coil, designed for a vented box, in a sealed box with high power, the woofer will run out of coil and you'll hear distortion. Because the condition that produces the distortion is symmetrical (coil leaves the gap in both directions), the distortion will be mostly odd-order, which sounds nasty. "Brap Brap Brap".

So, what's the conclusion?
Your power requirement and choice of box type determines whether a long coil (woofer with high Xmax) is necessary. Almost all woofers will work in a sealed or vented box, so long as the woofer's Qts is lower than your target Qtc. A woofer with a Qts higher than .707 will have a peak in its response NO MATTER WHAT KIND OF BOX YOU USE. The rest is a matter of compromising between box size, power required to hit a target SPL and required low frequency extension.

When you put the box in the car, the transfer function (car's frequency response) will be added to the response of the woofer. this will produce roughly a 12dB per octave increase in level as frequency is reduced starting at about 50 or 60 Hz, depending on the size of the car. A vented box will give you a big peak and a sealed box will not. If you have an EQ, then you can reduce the peak by reducing the power the amp has to provide at those frequencies. I think a vented box and EQ is always the best way to go, so long as you can afford the space and the EQ.

Things that dictate low frequency response:


Environment - usually, a vehicle's transfer function offers a HUGE boost in the sub-bass region. Typically, a 20dB natural boost @ 16Hz.

Enclosure - in a car, sealed will ALWAYS play lower than a vented enclosure. Typical sealed box woofers have no problem reaching down to 10Hz flat, in car.

Driver: Fs is NOT a huge determiner of LFE. Qt is. The higher the Q, the lower the woofer will play - always. The more moving mass, the lower the driver will play (relative to Q). Less BL product (relative to Q). Etc etc etc.

Excursion - will my driver have the ability to move enough air to play that low? Remember, in a sealed environment, or IB (etc), the lower the speaker plays, the more excursion is needed.


Cms, or compliance usually has VERY little to do with how low a speaker will play. Now, keep in mind, that the lower the Fs of a driver is, the lower the Q is - and remember, a high Q always plays lower. So, a low resonance speaker isn't always good.


There is NO way to look at driver specs, to know how low a driver will play. You can get a good idea by looking at the Q. But, the BIGGEST factor of LFE, is environment. And, unless you map your transfer function of your vehicle, you cannot say that X driver will play down to Y.

Would you agree with this statement?
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Old 06-07-2015   #18
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Default Re: Designed for sealed or ported...explained!

Andy I found it interesting that you said that the pressure inside the box is the greatest near the tuning frequency while at the same time the driver hardly moves. I have watched a CRX with a single 12 in a sonotube w/ port firing rear do +150db and to me it appeared that the speaker was hardly moving. I could never understand how a speaker that hardly moved could be so devastating.

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Old 06-07-2015   #19
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Default Re: Designed for sealed or ported

Quote:
Originally Posted by troubleshootn View Post
Things that dictate low frequency response:


Environment - usually, a vehicle's transfer function offers a HUGE boost in the sub-bass region. Typically, a 20dB natural boost @ 16Hz.

Enclosure - in a car, sealed will ALWAYS play lower than a vented enclosure. Typical sealed box woofers have no problem reaching down to 10Hz flat, in car.

Driver: Fs is NOT a huge determiner of LFE. Qt is. The higher the Q, the lower the woofer will play - always. The more moving mass, the lower the driver will play (relative to Q). Less BL product (relative to Q). Etc etc etc.

Excursion - will my driver have the ability to move enough air to play that low? Remember, in a sealed environment, or IB (etc), the lower the speaker plays, the more excursion is needed.


Cms, or compliance usually has VERY little to do with how low a speaker will play. Now, keep in mind, that the lower the Fs of a driver is, the lower the Q is - and remember, a high Q always plays lower. So, a low resonance speaker isn't always good.


There is NO way to look at driver specs, to know how low a driver will play. You can get a good idea by looking at the Q. But, the BIGGEST factor of LFE, is environment. And, unless you map your transfer function of your vehicle, you cannot say that X driver will play down to Y.

Would you agree with this statement?
No, I don't agree with that. The car is a constant and it boosts bass as frequency decreases. If X woofer will play Y frequency, it'll play Y frequency louder in the car than out of the car.
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Old 06-07-2015   #20
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Default Re: Designed for sealed or ported...explained!

Quote:
Originally Posted by Sine Swept View Post
Andy I found it interesting that you said that the pressure inside the box is the greatest near the tuning frequency while at the same time the driver hardly moves. I have watched a CRX with a single 12 in a sonotube w/ port firing rear do +150db and to me it appeared that the speaker was hardly moving. I could never understand how a speaker that hardly moved could be so devastating.
Because at that frequency, most of the sound comes from a port.

Think about swimming vs. flying. You can flap your arms all you want and you won't fly very far. Jump in the pool and flap your arms and you can swim pretty easily. The water provides greater resistance to the motion of your arms. The port in the box works the same way. It increases the acoustical impedance.
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Old 06-07-2015   #21
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Default Re: Designed for sealed or ported...explained!

subbed for info.
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Default Re: Designed for sealed or ported...explained!

So how will it impact sound if it is a sealed enclosure in a sealed trunk vs an open back enclosure in a sealed trunk. will it work in a similar way or is the trunk just a bass chamber?
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Default Re: Designed for sealed or ported...explained!

So when looking for an IB sub , the qts of the sub will be the qtc.
So IB subs look for QtC between .5 and .7
Obviously a trunk may raise q a bit so large vas numbers on sub help keep that number close to qts.

Awesome sticky btw. Well explained


Andy,
Would you care to elaborate on damping and itís effects on transients below .707
Itís tough to explain and think your eloquence would add nicely to this sticky.

Thanks in advance

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Last edited by oabeieo; 02-20-2019 at 12:28 AM..
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Default Re: Designed for sealed or ported

Quote:
Originally Posted by Andy Wehmeyer View Post
they're both right and they're both wrong.

Here's the deal...go get a cup of coffee...

The Qts of the speaker indicates the shape of the roll-off. A Qts of .707 is considered ideal for infinite baffle because provides the best compromise between low frequency extension and transient response. So, let's say you have a woofer and you want to build a sealed box. Let's also say you have a TARGET Qtc (total Q of the box and woofer) of .707 and your woofer has a Qts of .5

The box volume will be calculated to provide an "aid" to the speaker's suspension (makes the suspension stiffer) so that the Qtc will be raised to the target of .707. If the woofer has a very compliant suspension (low Q) then the box will have to contribute more restoring force, so it will have to be small. If the woofer has a stiff suspension (higher Q) then the box will have to contribute less restoring force, so it will have to be bigger. If the Qts of your woofer is higher than the target Qtc, then the target isn't achievable.

So, the Q of the woofer determines the volume of the box for a given target Qtc. The low frequency extension is determined by the woofer's resonance frequency, Fs. The Fs is also raised in the sealed box iin the same proportion as the increase in Qts to Qtc.

So, a woofer with a really low Qts and a low Fs is well suited to a small sealed box SO LONG AS THE VOICE COIL IS LONG. Sealed boxes require much more excursion from the speaker than vented boxes do. A woofer with a short coil is not well suited to a sealed box unless it will be driven with very low power.

A vented box is more complicated. It's a combination of a sealed box and an additional resonance. A low Q woofer designed for a sealed box will work just fine in a vented box, but the additional coil necessary for the sealed box will be wasted in the vented box and its mass makes for a less efficient system than a woofer with the same Q and less mass.

Anyway, the box volume for a vented box is calculated to provide a target response similar to the sealed box. Usually the volume is a bit bigger because we want a lower Qtc (which doesn't really exist in the vented arrangement, but we'll use it as an example).

The port is basically a speaker that's designed to play loudly at one note. Its response is a peak. We design the port to have a response that compliments the response of the woofer. We hear the sum of the output of the woofer and the output of the port. If we increase the tuning frequency, we have an area where the port's response and the woofer's response have a small gap--that creates a peak in the response in those frequencies. If we move the port to a lower frequency, we have a dip in the frequencies in between. Usually, the resonance frequency is chosen to extend the low frequency response as low as possible while maintaining flat response. For small bookshelf-style home speakers, a little bump is often helpful in providing some additional bass.

At the frequency where the box is tuned, the port plays, but the woofer hardly moves. This is because the acoustic impedance (pressure) inside the box is much higher. Yes, the pressure inside a ported box is HIGHER (at the tuned frequency) than in a sealed box. Below the frequency where the box is tuned, there is much LESS pressure than in a sealed box--hence the need for a subsonic filter. So, at the low frequencies where the box is tuned, the woofer doesn't move much, so we don't need a heavy long coil. That makes it easier to make a more efficient woofer, since we don't need so much moving mass.

OK, so what does all of this mean?

1. If you're going to use lots of power and a sealed box, then you need a woofer with a long coil.

2. If you're going to use a vented box, a woofer with a shorter coil will be fine.

3. If you use a woofer designed for a small sealed box (low Q) in a vented box, the box volume requirement will be small and the port frequency requirement will be low. That means the port will be very long and the box will be difficult to build because the port will be difficult to fit in the box.

4. If you use a woofer with a short coil, designed for a vented box, in a sealed box with high power, the woofer will run out of coil and you'll hear distortion. Because the condition that produces the distortion is symmetrical (coil leaves the gap in both directions), the distortion will be mostly odd-order, which sounds nasty. "Brap Brap Brap".

So, what's the conclusion?
Your power requirement and choice of box type determines whether a long coil (woofer with high Xmax) is necessary. Almost all woofers will work in a sealed or vented box, so long as the woofer's Qts is lower than your target Qtc. A woofer with a Qts higher than .707 will have a peak in its response NO MATTER WHAT KIND OF BOX YOU USE. The rest is a matter of compromising between box size, power required to hit a target SPL and required low frequency extension.

When you put the box in the car, the transfer function (car's frequency response) will be added to the response of the woofer. this will produce roughly a 12dB per octave increase in level as frequency is reduced starting at about 50 or 60 Hz, depending on the size of the car. A vented box will give you a big peak and a sealed box will not. If you have an EQ, then you can reduce the peak by reducing the power the amp has to provide at those frequencies. I think a vented box and EQ is always the best way to go, so long as you can afford the space and the EQ.


Thanks Andy very informative as always...

Can you summarize the key points for people not as clued up as you are...

Such as what is considered a very low QTS and whatís considered to be a high Xmas and whatís considered a low FS?

So essentially some sort of guide for sealed and ported based on numbers

Thanks Andy


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Old 02-20-2019   #25
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Default Re: Designed for sealed or ported...explained!

guys, andy doesnt come around here anymore

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